Light-emitting diodes (LEDs) are among the most efficient light sources currently available. Recently, active efforts are made to develop white LEDs by combining blue LED with phosphor or fluorescent material. The mainstream is a pseudo-white LED device which is based on a blue LED combined with a yellow light-emitting phosphor. Examples of the yellow light-emitting phosphor include Y3Al5O12:Ce, (Y,Gd)3(Al,Ga)5O12:Ce, (Y,Gd)3Al5O12:Ce, Tb3Al5O12:Ce, CaGa2S4:Eu, (Sr,Ca,Ba)2SiO4:Eu, and Ca-α-SiAlON:Eu.
The pseudo-white LED illuminating device is generally constructed by dispersing yellow light-emitting phosphor particles in a transparent resin to form a resin-bonded fluorescent material and placing the fluorescent material in front of a blue LED. Pseudo-white light is produced by wavelength converting a portion of light emitted by the blue LED and combing the output with the remaining light.
In this connection, JP-A 2007-150331 discloses a fluorescent material comprising garnet or the like, specifically a light-transmissive, homogeneous fluorescent material capable of fluorescent emission upon excitation by light emitted by a light-emitting component, and a light-emitting device comprising the fluorescent material. This fluorescent material is expected to have higher heat resistance and higher mechanical strength than the prior art resin-bonded fluorescent material comprising a powdered phosphor and a resin in which the phosphor is dispersed, and is also expected to have high durability against the heat that increases as the output of light emitting components increases in the future.
Such sintered fluorescent materials are prepared by formulating a powder raw material having the same composition of constituent elements as the final fluorescent material, shaping, machining, and sintering.